1. Field of the Invention
The present invention relates to a tunable demultiplexer, and a tunable laser, and more specifically, to an optical device having an optical deflector that deflects radially spreading light, which has a predetermined type of deflection pattern region and a peripheral region arranged with a boundary due to a difference in a doping structure of an upper cladding layer, and has a refractive index of a core layer that varies in the deflection pattern region by control of an external electrical (current or voltage) signal.
2. Discussion of Related Art
First, a tunable demultiplexer will be described.
In an optical communication system, especially, a wavelength division multiplexing (WDM)-based optical system, it is necessary to have functions such as multiplexing for gathering various wavelengths of beams into one place, and demultiplexing for distributing any beam over a specific place according to wavelength.
So far, these functions are implemented with various methods and constructions, such as using a prism, an integrated concave grating, or an arrayed waveguide grating.
Among these, constructions using the integrated concave grating and arrayed waveguide grating can be formed with an optical waveguide based on material such as silica, GaAs, InP, LiTaO3, and polymer, thereby facilitating integration and enhancing device capability. Thus, the constructions using the integrated concave grating and arrayed waveguide grating, such as the construction schematically shown in FIG. 1, are widely used.
Hereinafter, a demultiplexer using the conventional concave grating will be described with reference to FIG. 1. FIG. 1 is a schematic diagram showing a demultiplexer using the conventional concave grating.
While the demultiplexer is shown in FIG. 1, the device shown herein may also be used as a multiplexer since they are reciprocal in terms of incident beam and exit beam. Therefore, for the sake of brevity, only the demultiplexer will be described.
The waveguide type concave grating construction having the demultiplexing function described above is shown in FIG. 1. When beams having broadband optical signals are incident on an input waveguide 10 in the above construction, the beams propagate through the input waveguide 10 toward a grating 30 at a point A. The propagating beams are diffracted at the grating 30 and distributed into respective output waveguides 20 according to wavelength. Here, the distributed beams are always located on a locus referred to as a Rowland circle. A radius R of the concave grating is a diameter of the Rowland circle, and a point where the concave grating and the Rowland circle intersect is referred to as a pole.
The detailed construction and application regarding the conventional construction is disclosed in document 1 (“Monolithic Integrated Wavelength Demultiplexer Based On a Waveguide Rowland Circle Grown In InGaAsP/InP”, IEEE Journal of Lightwave Technology, vol. 16, no. 4, April 1998) and document 2 (Theory and Simulation of a Concave Grating Demultiplexer for Coarse WDM Systems” IEEE Journal of Lightwave Technology, vol. 20, no. 4, April 2002.)
The tunable demultiplexer according to the prior art will be described. When a concave grating-type tunable demultiplexer is used, a construction may be used where several input waveguides are fabricated and beams having broadband optical signals are incident into one of the input waveguides. In this case, the incident beam is located on the Rowland circle, and an incident angle of the incident beam is changed to tune the wavelength based on the grating formula. For example, in a case where, for the above construction, a beam is incident into one waveguide and beams having wavelengths λ1 to λn exit out of respective output waveguides, when a beam is incident into another waveguide, the wavelengths exiting out of the out waveguides are changed into λ′1 to λ′n, respectively.
In this construction, the position of the incident beam is selectively adjusted to change the wavelength, and thus there are problems in that several input waveguides are required, a wavelength is tuned discretely, and an amount of tunable wavelength is restrictive.
In another construction, beams are incident into an optical fiber instead of the input waveguide, and the position of the optical fiber is moved to operate the tunable demultiplexer. This construction is disclosed in detail in document 3 (“Tunable Planar Concave Grating Demultiplexer”, IEEE Photonic Technology Letter, vol. 8, no. 4, April 1996). Here, the position of the optical fiber can be continuously moved, so that a wavelength can be tuned continuously. However, there are problems in that the construction is not structurally stable due to spatial movement of the optical fiber, and a speed for tuning wavelength is low.
Further, an arrayed waveguide grating construction may be used in the tunable demultiplexer. In other words, various input waveguides are used. When the radiating beam is moved from one point to another point in the construction, a phase of the beam coupled to the input aperture in an input coupler varies and a wavelength of the beam exiting out of the output waveguide is also changed.
The construction using the arrayed waveguide grating is based on the same principle as the construction using the concave grating, so that owing to its discrete wavelength tuning characteristic, it is difficult to use as a demultiplexer having a continuous tunable wavelength characteristic.
Therefore, among the conventional tunable multiplexer/demultiplexer, the construction using several input waveguides is not useful due to its discrete wavelength tuning characteristic, and the construction using the optical fiber is not structurally stable due to movement of the optical fiber and it has a low tuning speed.
Next, a tunable laser will be described.
Recently, with the advent of new multimedia technology such as VOD and video conferencing, and the ongoing development of conventional data communications technology such as HDTV, LAN, and CATV, etc., a capacity of information is rapidly increasing. To maximize transfer efficiency, very high-speed IT network carriers use a wavelength division multiplexing transmission system that maximizes a bandwidth of existing optical fiber, instead of establishing a new optical fiber, which takes an enormous amount of time and money.
When the optical transmission system described above is used, optical devices having various functions are required. Here, a tunable wavelength semiconductor laser has attracted attention as a light source having the widest application range in the communication network.
The tunable wavelength semiconductor laser has mainly been used for optical measurement prior to the WDM transmission system, but after that, it is used as a substitute or emergency backup for fixed wavelength semiconductor lasers, and is in high demand as a light source of a reconfigurable optical add/drop multiplexer (ROADM). In addition, its applications have been expanded to various devices such as a packet switch, a wavelength converter, and a wavelength router of an optical network.
Many types of the tunable wavelength semiconductor laser have been proposed so far, and among them, a distributed Bragg reflector laser diode and an external cavity laser diode (ECLD) are widely used. A construction of the ECLD will be described in detail for comparison with the construction of the present invention.
The ECLD comprises a semiconductor laser and an external diffraction grating, and can obtain a continuous tunable wavelength characteristic by changing a diffraction condition with a spatial adjustment (rotational or translational movement) of the grating. The ECLD described above is widely used in a conventional measuring apparatus, due to its high output power, narrow bandwidth, and wide tuning range. However, it poses problems such as difficulty in properly arranging the semiconductor laser and the grating with respect to one another, mechanical vibration due to movement of the grating in wavelength tuning, and a wavelength shift due to aging of the location of a pivot point. In particular, a tuning speed is extremely low so that it is not feasible to use the ECLD in optical communication systems.
To address the low reliability and slow tuning speed of the ECLD construction described above, a construction in which the wavelength is electrically adjusted has been proposed.
In “Continuous tuning of an electrically tunable external-cavity semiconductor laser” Optics Lett., vol. 25, no. 16, pp. 1165–1167, August 2000, M. Kourogi, et al. proposed that wavelength tuning be performed using a beam deflection characteristic according to frequency variation of an external electrical signal, by inserting an acousto-optic modulator (AOM) between a laser diode and a grating rather than moving the grating. However, the aforementioned construction has a large AOM, a large insertion loss, and an extremely small frequency variation as much as only 2 nm.
Korean Patent Number No. 444,176 entitled “Optical deflector using electrical signal and tunable external resonator using the same” of K. Y. Oh, et al. proposes a tunable laser in a Littman external resonator comprising a semiconductor laser diode (or optical amplifier), a lens, a grating, and a reflective mirror. In the proposed construction, a deflector for deflecting a direction of a beam is inserted between the grating and the reflective mirror, and a current is injected to change a refractive index of a medium in the deflector and adjust an angle of incidence on the reflective mirror, to thus perform wavelength tuning.
In “Proposal of electrically tunable external-cavity laser diode” IEEE Photon Tech Lett., vol. 16, no. 8, pp. 1804–1806, August 2004, O. K. Kwon, et al. propose a construction in which a deflector is integrated into a semiconductor laser diode and a position of a waveguide beam varies due to a current injected into a deflector region to change an incident angle of a grating and thereby perform wavelength tuning. As described above, an external resonator-type tunable laser having an inserted or integrated deflector has a high speed and is structurally simple, but it is difficult to properly arrange the grating and the laser diode with respect to one another, and a device is large due to the lens and the grating.
Thus, the ECLD, or the conventional tunable laser, has a problem in that it is difficult to properly arrange the laser diode, the external grating, the lens, and the reflective mirror, and the device is large due to insertion of the additional optical parts.